PWM signals are being used in various fields, including the fields of motor control, power electronic control, light-emitting diode (LED) driving, etc. Among these PWM signals, PWM signals used in the field of power electronics are used chiefly in power supplies, and are used to control switching devices to generate target voltages. Recently, in response to the demands for the small size, high performance and high functionality of power supplies, the digital control of power supplies has been employed instead of a conventional analog control method.
In power supplies that are digitally controlled, a method of generating a PWM signal using a counter and a comparison circuit is used. More specifically, a value is counted up by inputting a clock signal into the counter, and the count value and a threshold value are compared with each other by inputting them to the comparison circuit, thereby generating a PWM signal. The counter is reset when a reset value set to a value larger than the threshold value is reached. The duty (logic “H” time ratio) of the PWM signal is changed by changing the threshold value, and the period of the PWM signal is changed by changing the reset value.
However, in the conventional technology, even when the threshold value is changed by only 1 in order to minutely change the duty cycle, the logic “H” time of the PWM signal is changed by one clock period. Accordingly, since the minimum unit of a change in duty cycle is large, it is difficult to minutely control the output power of a power supply and also it is impossible to construct a high-performance power supply. Moreover, control is performed chiefly using a low-resolution PWM signal and an analog feedback circuit together, and thus a problem arises in that it is difficult to generate a high-resolution PWM signal.
In order to solve the above-described problems, Korean Patent Application Publication No. 2008-0030928 discloses a PWM signal generation circuit and a power supply including the same. This technology is intended to provide a method of controlling the duty of a PWM signal that is capable of reducing the variation in duty cycle without increasing clock frequency. However, although this technology improves the resolution of a PWM signal over a wide duty cycle range by arbitrarily changing both the period and logic “H” time of an output PWM signal at time intervals of ½ of a clock period, it is difficult to generate a high-resolution PWM signal because there is no process of forming a control loop using only a PWM signal.